The present invention relates generally to voltage-to-current converters, and more specifically to linear voltage-to-current converters.
Phase lock loop (PLL) circuits and delay lock loop (DLL) circuits are commonly used in integrated circuits today. Example uses for these circuits include clock recovery in communications systems and clock signal alignment in digital systems.
PLLs and DLLs often incorporate a phase detector and a voltage controlled oscillator (VCO). The VCO generates an output signal with a phase and frequency that is a function of a control voltage. The phase detector measures the phase difference between an input signal and the output signal, and adjusts the control voltage of the VCO. The control voltage to the VCO represents a phase difference, or xe2x80x9cphase errorxe2x80x9d between the input signal and the output signal. When the phase error is large enough, the VCO changes the phase or frequency of the output signal to more closely match that of the input signal.
Examples of PLLs, DLLs, VCOs, and phase detectors are described in: Ian A. Young, Jeffrey K. Greason, and Keng L. Wong, xe2x80x9cA PLL Clock Generator with 5 to 110 MHz of Lock Range for Microprocessors,xe2x80x9d IEEE Journal of Solid-State Circuits, pp. 1599-1607, Vol. 27, No. Nov. 11, 1992; and Henrik O. Johansson, xe2x80x9cA Simple Precharged CMOS Phase Frequency Detector,xe2x80x9d IEEE Journal of Solid-State Circuits, pp. 295-299, Vol. 33, No. Feb. 2, 1998.
The phase detectors described in the above references exhibit a xe2x80x9cdead zonexe2x80x9d in the phase characteristic at the equilibrium point under certain conditions. The dead zone generates phase jitter in part because the VCO does not change the phase of the output signal when the phase error is within the dead zone. As the operating frequency of integrated circuits increases, PLLs, DLLs, and their associated VCOs and phase detectors are also operating faster, and the size of the dead zone becomes an important factor in the design of circuits.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate phase detectors and circuits that incorporate phase detectors.